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INTRODUCTION

Aggregates form the major part of the pavement

structures and are the prime materials used inpavement construction. Aggregates used in contest

of Nepal are natural deposits of sand and gravel and

crushed stones. Aggregates used in road construction

have to primarily bear load stresses occurring on

the roads and runways and have to resist wear due

to abrasive action of traffic. The performance of

aggregate layer depends on its intrinsic properties,

i.e. the particle shape, grading, composition and its

physical, mechanical and chemical properties.

Therefore, selection of aggregates depends on these

properties and specification set by national and

international norms. Works on aggregates and their

properties have been made earlier in different areas

by several authors (Tamrakar et al. 1999; Maharjan

and Tamrakar 2003; Thapaliya 2005; Dhakal et al.

2006; Maharjan and Tamrakar 2007; Tamrakar et

al. 2007). Tamrakar et al. (1999) suggested that the

mechanical properties of sandstones depended on

the rocktypes and content of calcium carbonate

cement, and were independent on deposition age of

rocks. Maharjan and Tamrakar (2003) evaluated

quality of siltstone samples from the Tistung

Bulletin of the Department of Geology, Tribhuvan University, Kathmandu, Nepal, Vol. 12, 2009, pp. 2942

Evaluation of quality of crushed-limestone and -siltstone

for road aggregates

*Shrawan Khanal and Naresh Kazi Tamrakar

Central Department of Geology, Tribhuvan University, Kirtipur, Kathmandu, Nepal

ABSTRACT

*Corresponding author:

E-mail address: [email protected]

Bulletin of the Department of Geology

Aggregates are important constituents of pavement structures. Performance of aggregate layers depends on particle shape,

grading and composition and their physical, mechanical and chemical properties. Careful study of these properties allows evaluation

of aggregates according to the international norms. There are huge rock outcrops surrounding the Kathmandu Valley. With growing

construction in and around the Kathmandu Valley, demand of aggregates has become so high that limited quarry sites within the

valley are not sufficient to meet the requirements. Therefore, search of rock outcrops from which suitable aggregates may be

quarried is sought to meet the current and future demands of aggregates suitable not only for the building and infrastructures but

also for the roads or transport networks in the country. This study aims in exploring and evaluating suitability of natural crushed-

rock aggregates in western Kathmandu, Nepal for unbound pavement structures.

Mainly three kinds of rocks were identified from the outcrops of the study area; siliceous limestone, crystalline limestone and

calcareous siltstone. Shape factor (F) ranges from 0.99 to 1.40 in most of the samples indicating their cubic to disc shape. Aggregates

have high roughness indices and moderate roundness indices. Flakiness indices of the test samples vary from 14% to 25% and

elongation indices from 75.49 to 90.79% indicating that the crushed samples yield very little flat and elongated particles, and hugeequant particles, which is very desirable for aggregates.

Dry density of samples ranges from 2495 to 2658 kg/m3. WAV varies from 0.20 to 0.67%. ACV ranges from 2226% and

aggregates are strong enough to resist against compressive load. LAA varies from 27 to 30 % and AIV ranges from 10 to 14%

showing that the aggregates have good hardness. The average CBR value is 61.12%. SSV ranges from 4.37 to 11.64%. All these

indices meet the BS standard, American standard and Nepal standard, and are acceptable for road base and sub base courses of

unbound pavement in roads.

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Formation for concrete aggregate and found that

the aggregates were appropriate for concrete

aggregates. Thapaliya (2005) studied limestones of

the Chandragiri Formation, southern Kathmandu,

and showed that the rocks were medium to high

strength, durable and suitable for both monumental

and construction purposes. Dhakal et al. (2006)

conducted freezethaw experiments on limestones,

sandstones, dolomite and schist, and concluded that

initiation and extension of cracks and subsequent

wearing and deterioration occurred relatively fasterin the rock having a high porosity, and the durability

of freezing-thawing was greatly influenced by

mineralogy. Maharjan and Tamrakar (2007)

evaluated quality of the river gravels for aggregates

and concluded that the majority of gravels had

diverse chemical groups, high durability and good

workability for road and concrete aggregates.

Tamrakar et al. (2007) analysed sandstones from

the Siwalik group and concluded that the strength

of the sandstones depended primarily on proportion

of void space in sandstones.

The Cambrian rocks of the Chandragiri Formation

(Stocklin 1980) are well distributed in the western

watershed of the Kathmandu Valley (Fig.1), in

Adeshwor area. Among the several quarries operated

in the peripheral part of the Kathmandu Valley,

natural crushed rock aggregates from Adeshwor

area are being supplied to the market without any

technical specification. Therefore, this study aims

in exploring and evaluating the suitability of natural

crushed-rock aggregates for unbound pavement

structure.

PAVEMENT STRUCTURES

In most of the roads the natural soil is seldom

strong enough to support the repeated application of

even modest wheel loads without significant

deformation. It is therefore necessary to interpose

between the wheel and soil structure to supplement

the natural strength of soil formation. The structure

thus constructed is called "pavement".

According to "Design and Evaluation of Rigid

and Flexible pavement TRR (1990)" pavement profiles

can be of two-layer, three-layer and four-layer profiles.

The design and performance of road pavements,

London (1991) traditionally classified pavements as:

(i) Flexible consisting of compacted stone beneatha bituminous surfacing. Modern flexible pavement

consists of three layers, i.e. bituminous surfacing,

road base and sub base as in Fig. 2a. The surfacing

is generally subdivided into a wearing course and

base course laid separately. The base and sub base

may also be laid in composite forms using different

materials and are designated upper and lower road

base or upper and lower sub bases as in Fig 2b.

(ii) Rigid or concrete of a concrete slab lays either

directly on the soil or on a shallow granular bed.

Concrete pavements normally consist of only concreteslab and the sub base, but a bituminous surfacing may

be added at a time of construction or later (Fig. 3).

Different types of pavement are constructed on

roads for safe and comfortable movements of vehicles

at the desired speed. The standard nomenclature of

the pavement used in Nepal consists of following

30

Fig. 1 A map showing location of the study area

S. Khanal and N.K. Tamrakar / Bulletin of the Department of Geology, Vol. 12, 2009, pp. 2942

China

Nepal

India

IchanguKathmandu

Sitapaila

Tikabhairav

Study area

Quarry site

27

45

27

35

85 15 85 30

Fig. 2 Components of flexible pavement: (a) Simple four-layer

pavement and (b) More details of the four-layer pavement.

Wearing courseSurfacing

Base course

Upper road baseRoad base

Lower road base

Upper sub baseSub base

Lower subbase

Sub grade Sub grade

(a) (b)

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layers according to the standard specification for road

and bridge works DOR (2001) (Fig. 4).

(i) Sub grade deals with the treatment of upper

layers of earthworks including preparation and surface

treatment of the formation, the addition of layers of

selected materials, the improvement of in situ materials

by addition and mixing of selected materials or by

addition or mixing of lime.

(ii) Sub base consists of granular materials, either

naturally occurring or crushed but they may be

granular materials stabilized with cement. Materials

used in sub base are gravel, sands, silty sand and

clayey sand.

(iii) Road base is the main structural component

of the road. Its function is to reduce the compressive

stresses in the sub grade and the sub base to an

acceptable level and to ensure that the magnitude of

the flexural stresses in the surfacing will not lead to

cracking. Dry single size stone free from clay, organic

or other deleterious matter is spread and then filledwith fines using vibratory compaction.

(iv) Surfacing includes a wide range of dense and

more open and textured material bound with bitumen

or tar binders.

Though these layers are the distinct constructive

layers in the pavements; structurally pavements are

divided in two groups on the basis of presence or

absence of them. Pavements are of two types; bound

and unbound (Fig. 4). In bound pavements aggregates

are bonded by cement or bituminous binders. Unbound

is used for sub bases or capping, but else where may

be used for base course. In the case of low volume

roads the whole structure is considered as unbound

pavements. A major difference between them is

absence of waring course and binders in case of

unbound pavement structure (Fig. 4).

GEOLOGICAL SETTING

The sampled site, AdeshwarKallabari ridge with

highest altitude 1606 m extending SENW meets

ultimately the Nagarjun Range in the north. The majordraining channels are the Jughe Khola in the north

and the Lupan Khola in the south of the ridge. The

Jughe Khola flows down from the NW direction

towards SE with its many small tributaries coming

31

Evaluation of quality of crushed-limestone and -siltstone for road aggregate

Fig. 4 Pavements structures (a) Bound pavement, typical flexible pavement construction layers and (b) Unbound pavement

(a) (b)

SubgradeSubgrade

Sub base course Sub base course

Base course Base course

Wearing course

Fig. 3 Component of concrete pavement

Bituminoussurfacing

Concrete slab

Concrete slab

Sub base Sub base

Sub grade Subgrade

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from north and south. The Lupan Khola flows from

the southern part of the ridge with its tributaries from

the north and the south. Mainly limestones and

siltstones are exposed at the quarry sites. Generally,beds strike NWSE and dip 40N (Fig. 5). Limestone

of the quarry is light grey to grey, medium- to coarse-

grained, slight to moderately weathered, well bedded,

and highly jointed with yellow to brown weathering

color (Fig. 6). Rocks exposed at the quarry area are

fresh while those exposed around the quarry are

weathered. At the top of the ridge the rocks are

weathered and only core stones are developed.

METHODOLOGY

The study area was surveyed on an scale of

1:25,000 to collect information on lithology, geological

structure and rock mass characteristics. Seven

representative block and fragmented samples from

the three different horizons were collected (Fig. 5).

Mineralogical and textural properties of rocks were

investigated by preparing thin sections and studying

under the polarizing microscope and analyzing the

images. Cores obtained from the block samples were

used for determination of physical and petrographic

properties. Fragmented samples were utilized for the

Fig. 5 A geological map showing structure of bedrocks distributed in study area, and the location of sampling sites.

Kallabari

Ramkot

Sitapaila

Hasantar

Buldol

Lupan

Khola

K.K.h.

Harisiddhi

Ichangu

Syuchatar

JugeKhola

1500

1600

1500

140

0

1400

1300

1500

1400

85

15

33

35

75

40 62

55

55

32

30

6555

80

40 75

8080

65

6986

86

45

60

32

64

34

30

52

7086

32

75

85o 15 00 85o

17 0085o

16 00

27o

4400

27o

4300

LEGEND

Attitude of bed

Fault

Watershed boundary

32Tistung Formation

Flood plain sediment

Alluvial fan

Fan-delta sediment

Chandragiri Limestone

Sopyang Formation

0 1 Km

N

Adeshwar

BR1

BR2

BR3BR4

BR5

BR7

BR6

BR3 Sampling point

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Fig. 6 Outcrop of a limestone at a quarry at the Right bank of the Jughe Khola facing north: (a) Limestones showing prominent

joints, and (b) Extension of limestone outcrop in the quarry site


determination of shape parameters using axial ratios

as well as image analysis, and of other mechanical

and chemical properties. Chemical analysis of the

samples for CaO and MgO was also made to find outCaO/MgO ratio. The data obtained from the

petrographic, physical, mechanical and chemical

analyses were used to evaluate the crushed-limestone

and siltstone for unbound pavements.

ROCK MASS CHARACTRISTICS

Discontinuities in the physical continuity of the

rocks include bedding surfaces, joints, faults and

foliations. Though there are four sets of joints in the

rocks, two major joints are prominent particularly in

limestones. Major joint sets are vertical (Table 1).

Joint spacing varies from 0.3 m to 3 m which means

joints are moderate to wide spaced. Two major joints

(a)

(b)

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are persistent and their roughness value ranges from

6 to 12 according to roughness scale of Barton and

Choubey (1977). Joint separation ranges from 0.1

mm to >20 mm. Spaces may be empty, partially filledor completely filled with silty clay and vegetation.

Most of the rocks on the outer surface fall in grade

II and III while inner excavating fall on grade IB.

Therefore, the rocks in study area are faintly weathered

to moderately weathered according to weathering

classification of Geological Society of London (1770).

Induration of the limestone ranges from H3H4 i.e.

indurated to strongly indurated.

PETROGRAPHIC ANALYSES

Altogether 33 thin sections were prepared. Thirteensections were coloured so that pore spaces in the

rocks could be studied. Thirteen stratified samples

were prepared to study the lateral variation in texture

and composition of the rocks, and other seven thin

sections were prepared from bulk sample area. Except

coloured sections, all other thin sections were stained

for separating calcite and dolomite by Dickson (1966)

method for staining carbonates. Massive purple to

royal blue colour on staining confirms that all the

sections contain ferroan calcite. No coloured spaces

in the thin sections except in some fracture shows

that limestones have exceptional pore spaces and low

porosity.

Composition of samples

Compositional analysis of the thin sections by

using the petrographic microscope and a scion image

analyzer provides that in most of the sections are

composed of calcite and quartz grains with few grains

of feldspar, mica and heavy minerals such as pyrite.

On the basis of percentage chemical composition

and grain size parameters according to Clark and

Walker (1977), rock samples have been classified in

to three sub groups, i.e., crystalline limestone, siliceouslimestone and calcareous siltstone (Table 2).

Crystalline limestone

Crystalline limestone contains more than 50%

carbonate mineral and show uniformity in crystal size

(Fig. 7a). Samples BR1 and BR3 are crystalline

varieties of limestone. Crystalline limestones are

highly indurated and have high unconfined

compressive strength. In outcrops, these limestones

are thick bedded, light grey, coarsely crystalline with

Table 1: Rock mass characteristics

Sampleno.

Weatheringgrade Discontinuities Dip dir./amount Persistency Spacing Width Roughness Infilling materials

Bedding 207/879 Continuous 0.21m 0.11mm 810 Silty clay

Joint 1 Vertical Contd. no infill 0.51.3m 15mm 68 silty clay+veg

Joint 2 100/83 Noncontinuous 0.31m 0.11mm 1012 silty clay+veg

Bedding 207/79 Continuous 0.31m 0.11mm 810 Silty clay

Joint 1 115/83 Contd. no infill 530cm 15mm 1012 silty clay+veg

Joint 2 60/39 Noncontinuous 13m 0.11mm 68 Silty clay

Bedding 357/76 Continuous 0.11m 20mm 1012 silty clay+veg

Joint 2 vertical Noncontinuous 530cm 0.11mm 810 clay

Joint 3 280/83 Noncontinuous 0.31m 3m

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well developed parallel lamination.

Siliceous limestone

Siliceous limestones constitute 5070% of the

total carbonate content. Generally, crystals are ofsand-sized grains, and are subhedral in shape (Fig.

7b). Sample BR2, BR6 and BR7 are siliceous varieties

of limestone. Siliceous limestones are moderately

indurated and have moderately strong to strong

unconfined compressive strength, and are thick to

thin bedded, grey, and medium-crystalline.

Calcareous siltstone

Calcareous siltstones (samples BR4 and BR5)

contain 1050% total carbonate content. Grain size

ranges from silt to that of the sand of anhedral shape

(Fig. 7c). Calcareous siltstones are moderately

indurated, and are correlated with strength of

moderately strong to strong rocks (Maharjan and

Tamrakar, 2003). In the field these rocks are dark

grey, fine-grained, argillaceous, and cross-laminated.

Most sections show heterogeneous distribution of

quartz grains interlocked with calcite. Quartz grains

show polycrystalline nature under the plane polarized

light and sutured contacts, which show that quartz

grains are partially recrystallized.

Shape analysis of aggregates

The aggregates were studied for their shape factor

(F), sphericity (y), roughness index (Ru), roundness

index (Rn), flakiness index (FI), and elongation index

(EI). The results of the analyses are summarized in

Table 3.

Shape factor and Sphericity

Based on the longest (a), intermediate (b) and

shortest (c) axes of each grain, the shape was quantified

in terms of flatness ratio (p = c/b) and elongation

ratio (q = b/a). The p and q range from 0.65 to 0.79

and 0.57 to 0.71, respectively. Shape factor (F) is the

ratio of p/q and ranges from 0.99 to 1.40 in most of

the samples indicating their cubic to disc shape.

However the plot of flatness and elongation ratios

indicate that the most of the aggregates are cubic

(Fig. 8).

The Aschenbrenners working sphericity (f) was

calculated as:

Y = {12.8 (p2q)1/3}/{1+p(1+q)+6(1+p2(1+q2))}

Sphericity ranges from 0.82 to 0.90 (Table 3) and

Table 2: Results of the petrographic analysis of limestone and siltstone

Q F M H Homogenity Microfabric

BR1 78 14 2 5 1 0.032 0.132 Subhedral HI Homogenous Microlaminae Crystalline

limestone

BR3 85 10 1 3 2 0.0820.130

Elongated

and // to

lamination HI Homogenous Microlaminae Crystalline

limestone

BR6 55 40 3 2 1 0.130.205Anhedral tosubhedral MI Homogenous Microlaminae Siliceous

limestone

BR7 52 45 2 1 1 0.0820.105 Anhedral MI Heterogenous Microlaminae Siliceous

limestone

Massive/

polycrystalline

Massive/

polycrystalline

Massive/

polycrystalline

MI Heterogenous Calcareous

siltstone

3 1 0.0670.085 AnhedralBR5 50 45 2

Anhedral MI Heterogenous Calcareous

siltstone

MI Heterogenous Siliceous

limestone

BR4 40 48 5 3 4 0.0250.05

2 1 0.0750.202 AnhedralBR2 55 40 2

Shape Induration

Microstructures

ClassificationSample % Calcite

% Siliciclasts

Size (mm)

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(a)

0.24 mm

0.24 mm

Fig. 7 Photomicrographs of limestone; (a) Crystalline limestone lacking original texture, (b) Siliceous limestone (stained) showing

lamina and (c) Calcareous siltstone in which interlocked and elongate quartz grains

0.24 mm(b)

(c)

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shows that it possesses high sphericity.

Roughness and roundness

Roughness and roundness indices of Janno (1998)

were measured using NIH Scion image analyzer and

calculated using following relations:

Roughness index (Ru) = P/Pc

Roundness index (Rn) = 4pA/P2

where, A is area of particle, P is perimeter, and

Pc is a convex perimeter.

For a smooth material, the roughness index (Ru)

is equal to unity. As roughness increases, Ru also

increases. Increase in Ru shows that the aggregates

have sufficient surface roughness and have good

workability (Kaplan 1961; Neville 1996).

The roundness index (Rn) of a perfect circle is

100. As a material becomes angular, Rn decreases

(Janno 1998). Rounded aggregates produce

significantly higher permanent deformation than theangular ones. Internal frictional angle increases with

the increasing angularity and surface roughness (Holtz

and Kovacs 1981).

Roughness index ranges from 1.28 to 1.40 whereas

roundness index of the aggregates here varies from

52.65 to 54.77 (Table 3), which shows that aggregates

have high roughness and moderate roundness indices.

Flakiness and elongation index

Flakiness and elongation indices were determined

using the methods of determining particle shape by

BS 812 (BSI 1989). During FI test, aggregate sample

was sieved through different sieve sizes, at least 200

pieces of each fraction were taken, weighed and

allowed to pass through the selected slot-size which

had width equal to 0.6 times the mean dimension.

The fraction of samples passing through the thickness

gauge was weighed and FI was calculated as below:

Flakiness index (FI) = (WT/W) 100 (%)

where, WT is the weight of the aggregate passing

through the slot and W is the total weight of the test

sample.

0 0.2 0.4 0.6 0.8

1.0

0.8

0.6

0.4

0.2

1.0

Oblate Biaxial

Flatness Ratio (p = c/b)

Prolate

Biaxial

Equidimensional

Discs

B

lades

Rods

Spherocubes

0.9

0.8

0.7

0.6

0.50.4

0.2

ProlateBlades

O

blateBlades

F=3.0F

=1.5F

=1.0

F=0.87

F=0

.33

ProlateTriaxial

Obla

teTriaxia

l

ElongationRatio(q

=b/a)

Fig. 8 Shape diagram indicating mean forms of aggregate samples

Table 3: Results of the shape analysis of aggregate

Sample no.

Elongation

ratio(q)

Flatness

ratio (p)

Shape factor

(p/q)

Sphericity

(y)

Roughness

(Ru)

Roundness

(Rn) FI % EI %

BR1 0.69 0.73 1.1 0.86 1.36 54.47 25.03 75.49

BR2 0.66 0.77 1.34 0.86 1.28 54.77 22.37 90.79

BR3 0.7 0.79 1.16 0.90 1.28 53.33 22.06 76.03

BR4 0.71 0.65 0.99 0.85 1.31 52.81 20.93 83.92

BR5 0.65 0.69 1.1 0.85 1.32 53.3 14.74 89.95

BR6 0.7 0.72 1.06 0.87 1.32 53.51 1.61 86.68

BR7 0.57 0.72 1.4 0.82 1.4 52.65 16.02 87.61

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During elongation index test, aggregate was

allowed to pass through the length gauge whose

longest dimension was greater than 1.8 times their

mean dimension. The fraction, which did not passthrough the gauge, was weighed and EI was calculated as:

Elongation index (EI) = (WL/W) 100 (%)

where, WL is weight of aggregate retained in the

length gauge and W is the total weight of test sample.

Low percentage of FI shows that aggregate

contains only a few flat grains and high percentage

of EI indicates the presence of only small number of

elongated grains. Flakiness index of the test samples

varies from 14% to 25% and elongation index varies

from 75.49 to 90.79% (Table 3) indicating that the

samples are not so flat and elongated.

ANALYSES OF PHYSICAL AND

MECHANICAL PROPERTIES

The aggregates were studied for their dry density,

water absorption value (WAV), aggregate crushing

value (ACV), aggregate impact value (AIV), Los

Angeles Abrasion (LAA) and California Bearing

Ratio (CBR).The results of these analyses are

summarized in Table 4.

Dry density and water absorption

The physical properties of aggregate such as dry

density and water absorption value were determined

for cylindrical samples according to caliper and

saturation method (ISRM 1979).

Dry density of samples ranges from 2495 Kg/m3

to 2658 Kg/m3 with an average value of 2600 Kg/m3.

Water absorption value (WAV) ranges from 0.20 to

0.67% which is less than that of the standard (Table

4). This shows that samples have low effective

porosity.

Crushing and impact values

Aggregate crushing value (ACV) and Aggregate

impact value (AIV) were determined using a

compression testing machine and hammer respectively

following ASTM (1979). ACV provides the relativemeasure of resistance to crushing under the gradually

applied compressive load while AIV is the resistance

of the stones to fracture under repeated impact. ACV

and AIV were calculated using the equations:

ACV = (W2/W1) 100 (%)

AIV = (W3/W1) 100 (%)

where, W1 is the total weight of the dry sample

(grams), W2 is the weight of the aggregate passing

4.75 mm sieve, and W3 is the weight of an aggregatepassing on 2.36 mm.

ACV ranges from 20 to 30% (Table 4) which

indicates that aggregates are strong, as a crushed

fraction is low. The results shows that the AIV lies

Table 4: Results of the physical, mechanical and chemical analysis of limestone aggregate

Sample

no.

WAV% Dry density

(Kg/m3)

LAA % AIV% ACV% SSV% Compositional group

BR1 0.36 2594 30.3 10.04 24.16 8.79 Crystalline limestone

BR2 0.29 2615 30.0 11.08 25.83 10.32 Siliceous limestone

BR3 0.20 2652 28.4 12.17 25.16 8.57 Crystalline limestone

BR4 0.52 2536 29.2 12.12 26.50 10.1

61.00

Calcareous siltstone

BR5 0.26 2626 30.2 14.05 26.33 6.81 Calcareous siltstone

BR6 0.31 2605 28.0 14.12 26.16 4.37 Siliceous limestone

11.45 22.00 11.64 Siliceous limestoneBR7 0.68 2567 26.8

CBR %

61.23

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between 10 to 20%, which is under the standard value

of ASTM, 10-20%, BS < 20% and NRS (NRS)

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